Northstar Engines and System Technical Discussion Discussion, Diagnostic Troubleshooting in Cadillac Engine Discussion; My car is no longer making any noise from bad gas. I have done a couple refills with some Shell ...

Diagnostic Troubleshooting

My car is no longer making any noise from bad gas. I have done a couple refills with some Shell V-power 93 Octane. Fixed my coolant leak; put in new radiator, changed hoses and stat. Also changed the drive belt and one of my torque struts. Checked and cleaned plugs.

Now what I am looking at is this: RPMs flucuate every couple of seconds between 700-1000 at idle when the A/C is on. RPMs are steady when A/C is off. I decided to monitor my egr valve to make sure it was ok. Thru the PCM Data mode, here is what I had:

Re: Diagnostic Troubleshooting

Not EGR problem.

It is normal for the AC compresssor to change RPMs (although I don't know if 700-1000 is too much). Even though the AC function is constantly on, the compressor cycles in and out depending on the temperature of the evaporator (every several seconds). When the compressor cycles, the engine needs some time to adjust the rpms.

You can verify that by monitoring the AC solenoid indicator (or by looking at the compressor clutch to see if its engagement corresponds to the rpm changes).

I think you are worrying for nothing.

Gas Gauge in PCM mode. Yes, I notice the wild bouncing too. I assume that in this mode the integration (smoothing) of the sending units signal is disabled. So, you can actually see in real time the position of the float in the gas tank as the gas is swishing around the tank. Maybe Bbobybski can confirm that (but that is what I think happens).

Re: Diagnostic Troubleshooting

Thanks for the reply. I understand that when the compressor engages that the load will change the RPMs. But I have never noticed it flucuate every few seconds. This happens when the interior is hot, so I would think the compressor would stay engaged longer. Where can I monitor the AC solenoid indicator? I could not find it in the OBDI.

Re: Diagnostic Troubleshooting

Your car is little different than mine. On mine, while I'm in the diagnostic mode, one of the Climate Control display segments shows the status of the Compressor Clutch.

Maybe someone with info on you car can tell you which segment it is.

You can always just open the hood and look at the compressor clutch. In most cars the
clutch plate is at the end of the pulley. So, if the end of the compressor pulley is spinning,
the clutch is engaged. If the belt is turning but the end of the pulley is not spinning then
the compressor is off. And as you are watching the pulley, you should be able to hear the change in engine speed.

Re: Diagnostic Troubleshooting

Here is an update:

Was still running a little rough. I had taken the bosch platinum+2s out and cleaned/replaced them about 2 weeks ago. Well, I decided with all this fluctuating rpm at idle, shuddering when hitting third gear at steady accelleration and bad mileage that I would just replace the plugs with oem ac delcos. When I took the plugs out, the one from #6 cylinder was still shiny and clean like it hadn't been used. I had already bought the delcos, so I put them in. The wires were under warranty. I had taken them in also and received a free replacement set. Might as well be on the safe side. After I got it all together, I started it up. I took the #6 wire off and there was no spark. It is on the same coil as #7. I checked #7 and it was still sparking. Next, I did a cylinder balance check through the pcm. I could notice no difference when I shut down #6 and only a very faint difference when I did #7. So...I went and got a new coil pack.

Whoa! Oh yeah! This baby is running smooth now. Problem was the coil pack. I am almost positive the one I took off was the original. As for the Bosch's...I still can't say they are bad for the north*. But I am going to stick with delcos with this Eldy.

Re: Diagnostic Troubleshooting

Thanks for letting us know what the culprit was. It helps other people in diagnosing thier problems.

Curious how the bad coil pack rough idle seemed to be affected by the AC being turned on. That is strange... Unless, since the engine wasn't firing on all cylinders, any load (like AC) made it change RPMs more drastically than it normally would.